In short, yes.
If a seller (like me) has not trademarked his/her brand, and has not registered with Amazon's Brand Registry, then the listing can be hijacked.
Of course, hijackers will only considers taking listings with many five-star reviews extending over several years.
Speaking from the experience - listing can be hijacked even if it has a brand registered trademark. These hijackers, rumored, are bribing amazon insiders to change trademark on the listing and gain full editing access to it after change is made.
